Among semiconductor elements, power elements for supplying power have a relatively large amount of heat generation. Therefore, as a substrate mounted with the power element, a substrate for a power module with a heat sink in which a first metal plate made of Al (aluminum) is bonded on a ceramic substrate made of, for example, AlN (aluminum nitride) or Si3N4 (silicon nitride), and a heat sink is connected to an opposite side of the substrate via a second metal plate made of Al (aluminum) is used.
In the substrate for a power module with a heat sink, the first metal plate is formed as a circuit layer, and a semiconductor chip serving as a power element is mounted on the first metal plate via a solder material.
Conventionally, the substrate for a power module with a heat sink is produced according to the following procedure as described in, for example, Patent Document 1.
At first, a first metal plate is laminated on one surface of a ceramic substrate via a brazing filler metal, and a second metal plate is laminated on the other surface of the ceramic substrate via a brazing filler metal. The ceramic substrate is then pressed with a predetermined pressure in the lamination direction and heated, to bond the ceramic substrate, the first metal plate, and the second metal plate (ceramic substrate bonding step).
Then, a heat sink is laminated via a brazing filler metal on a surface of the second metal plate, which is on an opposite side to the ceramic substrate. This is pressed with a predetermined pressure in the lamination direction and heated, thereby bonding the second metal plate with a heat sink (heat sink bonding step).
That is to say, the substrate for a power module with a heat sink has been produced heretofore by the ceramic substrate bonding step and the heat sink bonding step performed subsequent thereto.
At the time of brazing the heat sink to the second metal plate, a brazing filler metal foil of Al—Si alloy containing Si in an amount of 7.5% by mass or more is frequently used in order to set the brazing temperature low. In the Al—Si alloy containing a relatively large amount of Si, it has been difficult to produce a foil material by rolling or the like due to insufficient ductibility.
Moreover, a brazing filler metal foil is arranged between the heat sink and the second metal plate, and these are pressed in the lamination direction and heated. However, the brazing filler metal foil, the heat sink, and the second metal plate need to be arranged and laminated so that the position of the brazing filler metal foil is not misaligned at the time of pressing.
Furthermore, when the brazing filler metal foil is used, an oxide layer is present on four surfaces, that is, surfaces of the second metal plate and the heat sink, and opposite surfaces of the brazing filler metal foil, in an interface between the second metal plate and the heat sink, and hence, the total thickness of the oxide film tends to increase.